Date Published: October 8, 2018
Publisher: Public Library of Science
Author(s): Jakub Krejčí, Michal Botek, Andrew J. McKune, Laurent Mourot.
To find the shortest, acceptable stabilization period before recording resting, supine ultra-short-term Ln RMSSD and heart rate (HR).
Thirty endurance-trained male athletes (age 24.1 ± 2.3 years, maximal oxygen consumption (VO2max) 64.1 ± 6.6 ml·kg-1·min-1) and 30 male students (age 23.3 ± 1.8 years, VO2max 52.8 ± 5.1 ml·kg-1·min-1) were recruited. Upon awaking at home, resting, supine RR intervals were measured continuously for 10 min using a Polar V800 HR monitor. Ultra-short-term Ln RMSSD and HR values were calculated from 1-min RR interval segments after stabilization periods from 0 to 4 min in 0.5 min increments and were compared with reference values calculated from 5-min segment after 5-min stabilization. Systematic bias and intraclass correlation coefficients (ICC) including 90% confidence intervals (CI) were calculated and magnitude based inference was conducted.
The stabilization periods of up to 30 s for athletes and up to 60 s for students showed positive (possibly to most likely) biases for ultra-short-term Ln RMSSD compared with reference values. Stabilization periods of 60 s for athletes and 90 s for students showed trivial biases and ICCs were 0.84; 90% CI 0.72 to 0.91, and 0.88; 0.79 to 0.94, respectively. For HR, biases were trivial and ICCs were 0.93; 0.88 to 0.96, and 0.93; 0.88 to 0.96, respectively.
The shortest stabilization period required to stabilize Ln RMSSD and HR was set at 60 s for endurance-trained athletes and 90 s for university students.
Analysis of heart rate variability (HRV) has provided a non-invasive method for evaluating cardiac autonomic regulation [1,2]. In sports science, important applications of HRV analysis include monitoring responses to training loads [3–7], detection of overreaching signs [8,9], and HRV-guided training [10–13]. It is necessary for HRV analysis to record RR intervals for a sufficient period. A 5-min recording period was recommended as the standard for short-term HRV analysis . In addition, a resting recording should be started when RR intervals have stabilized. Therefore, a stabilization period is required before the start of the recording. However, guidelines  did not provide recommendations for choosing the stabilization period. Various stabilization periods have been used in the literature: 0 min [10,11], 1 min [6,9], 2 min , 3 min , 4 min , and 5 min . Alternatively, the stabilization period was automatically selected based on stable heart rate (HR) detection . It is clear that the stabilization period is not sufficiently standardized in sports science literature.
Athletes, compared with the students, demonstrated most likely higher VO2max, possibly higher both Ln RMSSD and RMSSD, and very likely lower HR (Table 1). For athletes, the shortest stabilization period that met our requirements described in the method section was 1.0 min for both Ln RMSSD (Table 2) and RMSSD (Table 3) and 0.5 min for HR (Table 4). Therefore, for the simultaneous measurement of Ln RMSSD and HR, the shortest stabilization period was 1.0 min. Stabilization periods from 1.5 to 4.0 min also met the requirements. For students, the shortest stabilization period was 1.5 min for both Ln RMSSD (Table 2) and RMSSD (Table 3) and 1.0 min for HR (Table 4). For the simultaneous measurement of Ln RMSSD and HR, the shortest stabilization period was 1.5 min. Stabilization periods ranging from 2.0 to 4.0 min also met the requirements.
The primary finding of this study was that a shortened RR interval measurement, consisting of a 1-min stabilization period and 1-min recording period, is an acceptable substitution for the traditional procedure that uses a 5-min stabilization period and 5-min recording period in endurance athletes. The shortened measurement protocol saves up to 80% of the time and it is proposed that the reduced time commitment will improve the attractiveness of HRV analysis for athletes, who require guided training load based on HRV analysis on daily basis [14,15] or at least three times a week [6,16].
When ultra-short-term (60 s) recordings are used to calculate Ln RMSSD and HR in the supine position, the minimal stabilization period required to stabilize both indexes was 60 s for endurance athletes but 90 s for university students.